Crate async_broadcast

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Expand description

Async broadcast channel

An async multi-producer multi-consumer broadcast channel, where each consumer gets a clone of every message sent on the channel. For obvious reasons, the channel can only be used to broadcast types that implement Clone.

A channel has the Sender and Receiver side. Both sides are cloneable and can be shared among multiple threads.

When all Senders or all Receivers are dropped, the channel becomes closed. When a channel is closed, no more messages can be sent, but remaining messages can still be received.

The channel can also be closed manually by calling Sender::close() or Receiver::close().

§Examples

use async_broadcast::{broadcast, TryRecvError};
use futures_lite::{future::block_on, stream::StreamExt};

block_on(async move {
    let (s1, mut r1) = broadcast(2);
    let s2 = s1.clone();
    let mut r2 = r1.clone();

    // Send 2 messages from two different senders.
    s1.broadcast(7).await.unwrap();
    s2.broadcast(8).await.unwrap();

    // Channel is now at capacity so sending more messages will result in an error.
    assert!(s2.try_broadcast(9).unwrap_err().is_full());
    assert!(s1.try_broadcast(10).unwrap_err().is_full());

    // We can use `recv` method of the `Stream` implementation to receive messages.
    assert_eq!(r1.next().await.unwrap(), 7);
    assert_eq!(r1.recv().await.unwrap(), 8);
    assert_eq!(r2.next().await.unwrap(), 7);
    assert_eq!(r2.recv().await.unwrap(), 8);

    // All receiver got all messages so channel is now empty.
    assert_eq!(r1.try_recv(), Err(TryRecvError::Empty));
    assert_eq!(r2.try_recv(), Err(TryRecvError::Empty));

    // Drop both senders, which closes the channel.
    drop(s1);
    drop(s2);

    assert_eq!(r1.try_recv(), Err(TryRecvError::Closed));
    assert_eq!(r2.try_recv(), Err(TryRecvError::Closed));
})

§Difference with async-channel

This crate is similar to async-channel in that they both provide an MPMC channel but the main difference being that in async-channel, each message sent on the channel is only received by one of the receivers. async-broadcast on the other hand, delivers each message to every receiver (IOW broadcast) by cloning it for each receiver.

§Difference with other broadcast crates

  • broadcaster: The main difference would be that broadcaster doesn’t have a sender and receiver split and both sides use clones of the same BroadcastChannel instance. The messages are sent are sent to all channel clones. While this can work for many cases, the lack of sender and receiver split, means that often times, you’ll find yourself having to drain the channel on the sending side yourself.

  • postage: this crate provides a broadcast API similar to async_broadcast. However, it:

    • (at the time of this writing) duplicates futures API, which isn’t ideal.
    • Does not support overflow mode nor has the concept of inactive receivers, so a slow or inactive receiver blocking the whole channel is not a solvable problem.
    • Provides all kinds of channels, which is generally good but if you just need a broadcast channel, async_broadcast is probably a better choice.

  • tokio::sync: Tokio’s sync module provides a broadcast channel API. The differences here are:

    • While this implementation does provide overflow mode, it is the default behavior and not opt-in.
    • There is no equivalent of inactive receivers.
    • While it’s possible to build tokio with only the sync module, it comes with other APIs that you may not need.

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